1. Field of the Invention
This invention relates generally to the field of polycrystalline diamond compact (PDC) drilling bits. More specifically, this invention relates to PDC bits which drill a hole through earth formations where the drilled hole has a larger diameter than the xe2x80x9cpass-throughxe2x80x9d diameter of the drill bit.
2. Description of the Related Art
Drill bits which drill holes through earth formations where the hole has a larger diameter than the bit""s pass-through diameter (the diameter of an opening through which the bit can freely pass) are known in the art. Early types of such bits included so-called xe2x80x9cunderreamersxe2x80x9d, which were essentially a drill bit having an axially elongated body and extensible arms on the side of the body which reamed the wall of the hole after cutters on the end of the bit had drilled the earth formations. Mechanical difficulties with the extensible arms limited the usefulness of underreamers.
More recently, so-called xe2x80x9cbi-centeredxe2x80x9d drill bits have been developed. A typical bi-centered drill bit includes a xe2x80x9cpilotxe2x80x9d section located at the end of the bit, and a xe2x80x9creamingxe2x80x9d section which is typically located at some axial distance from the end of the bit (and consequently from the pilot section). One such bi-centered bit is described in U.S. Pat. No. 5,678,644 issued to Fielder, for example. Bi-centered bits drill a hole larger than their pass through diameters because the axis of rotation of the bit is displaced from the geometric center of the bit. This arrangement enables the reaming section to cut the wall of the hole at a greater radial distance from the rotational axis than is the radial distance of the reaming section from the geometric center of the bit. The pilot section of the typical bi-centered bit includes a number of PDC cutters attached to structures (xe2x80x9cbladesxe2x80x9d) formed into or attached to the end of the bit. The reaming section is, as already explained, typically spaced axially away from the end of the bit, and is also located to one side of the bit. The reaming section also typically includes a number of PDC inserts on blades on the side of the bit body in the reaming section.
Limitations of the bi-centered bits known in the art include the pilot section being axially spaced apart from the reaming section by a substantial length. FIG. 1 shows a side view of one type of bi-center bit known in the art, which illustrates this aspect of prior art bi-center bits. The bi-center bit 101 includes a pilot section 106, which includes pilot blades 103 having PDC inserts 110 disposed thereon, and includes gauge pads 112 at the ends of the pilot blades 103 axially distant from the end of the bit 101. A reaming section 107 can include reaming blades 111 having PDC inserts 105 thereon and gauge pads 117 similar to those on the pilot section 106. In the bi-center bit 101 known in the art, the pilot section 106 and reaming section are typically separated by a substantial axial distance, which can include a spacer or the like such as shown at 102. Spacer 102 can be a separate element or an integral part of the bit structure but is referred to here as a xe2x80x9cspacerxe2x80x9d for convenience. As is conventional for drill bits, the bi-center bit 101 can include a threaded connector 104 machined into its body 114. The body 114 can include wrench flats 115 or the like for make up to a rotary power source such as a drill pipe or hydraulic motor.
An end view of the bit 101 in FIG. 1 is shown in FIG. 2. The blades 108A in the pilot section and the blades 111B in the reaming section are typically straight, meaning that the cutters 110 are disposed at substantially the same relative azimuthal position on each blade 108A, 111B. In some cases the blades 108A in the pilot section 106 may be disposed along the same azimuthal direction as the blades 111B in the reaming section 110.
Prior art bi-center bits are typically xe2x80x9cforce-balancedxe2x80x9d; that is, the lateral force exerted by the reaming section 110 during drilling is balanced by a designed-in lateral counterforce exerted by the pilot section 106 while drilling is underway. However, the substantial axial separation between the pilot section 106 and the reaming section 110 results in a turning moment against the axis of rotation of the bit, because the force exerted by the reaming section 110 is only balanced by the counterforce (exerted by pilot section 106) at a different axial position. This turning moment can, among other things, make it difficult to control the drilling direction of the hole through the earth formations.
Still another limitation of prior art bi-centered bits is that the force balance is calculated by determining the net vector sum of forces on the reaming section 110, and designing the counterforce at the pilot section 106 to offset the net vector force on the reaming section without regard to the components of the net vector force originating from the individual PDC inserts. Some bi-center bits designed according to methods known in the art can have unforeseen large lateral forces, reducing directional control and drilling stability.
One aspect of the invention is a bi-center drill bit which includes a body having pilot blades and reaming blades affixed to the body at azimuthally spaced apart locations. The pilot blades and the reaming blades have a plurality of polycrystalline diamond compact (PDC) cutters attached to them at selected positions along each of the blades. In one example of the invention, the pilot blades form a pilot section having a length along an axis of the bit which is less than about 80 percent of a diameter of a pilot section of the bit. In one example of this aspect of the invention, the total make-up length of the bit, including the length of the pilot section and a reaming section formed from the reaming blades is less than about 133 percent of the drill diameter of the bit.
In another aspect of the invention, selected ones of the pilot blades and reaming blades on a bi-center bit are formed into corresponding single (unitary) spiral structures to improve drilling stability of the bit. Selected ones of the reaming blade and pilot blades can be formed as spirals, where the azimuthal position of the cutters on each such spiral blade is different from that of the other cutters on that blade.
In another aspect of the invention, the shapes and positions of the blades, and the positions of the PDC cutters thereon of a bi-center bit are selected so that the lateral forces exerted by the reaming section of the bit and by the pilot section of the bit are balanced as a single structure, whereby the forces exerted by each of the PDC inserts are summed without regard to whether they are located on the reaming section or on the pilot section. These forces are in one example preferably balanced to within 10 percent of the total axial force exerted on the bit.
In another aspect of the invention, the center of mass of the a bi-center drill bit is located less than about 2.5 percent of the drilled diameter of the bit away from the axis of rotation (longitudinal axis) of the drill bit.
In another aspect of the invention, a bi-center drill bit includes drilling fluid discharge orifices (xe2x80x9cjetsxe2x80x9d) in the reaming section of the bit which are oriented so that their axes are within about 30 degrees of normal to the axis of the bit.
In another aspect of the invention, a bi-center bit includes reaming blades which are shaped to conform to whichever is radially least extensive, with respect to the longitudinal axis of the bit, at the azimuthal position of the particular blade, either a pass through circle or a drill circle. The drill circle and the longitudinal axis are substantially coaxial. The axis at the pass-through circle is offset from the longitudinal axis and defines an arcuate section wherein the pass-through circle extends laterally from the longitudinal axis past the drill circle. The leading edge cutters on the reaming blades are, as a result of this selected shape of the reaming blades, located radially inward of the trailing edge of the reaming blades with respect to the pass through circle where the reaming blades conform to the drill circle (in the arcuate section). This provides that the drill bit can pass through an opening having a diameter of about the pass-through diameter, for example casing in a wellbore, but can also drill out casing cementing equipment in a wellbore without sustaining damage to the leading edge cutters on the reaming blades.
Another aspect of the invention is a bi-center drill bit comprising a body having pilot blades and reaming blades affixed to the body at azimuthally spaced apart locations. The pilot blades and reaming blades having polycrystalline diamond compact (PDC) cutters attached to them at selected positions along each of the blades. The pilot blades have additional cutters attached to them at locations which are proximate to a circle defined by precessing the pass-through axis of the bit about the longitudinal axis of the bit. In one example, the additional cutters are tungsten carbide cutters, PDC cutters or diamond cutters. In one example, the side rake or the back rake angle of the cutters proximate to the circle is changed. In another example, additional cutters can be provided proximate to the circle by adding a row of cutters on thickened blade portions proximate to the circle.
Another aspect of the invention is a method for drilling out a casing having float equipment therein. The method includes rotating in the casing a bi-center drill bit having pilot blade and reaming blades thereon at azimuthally spaced apart locations. The blades have PDC cutters thereon. The reaming blades are shaped to conform to whichever is radially least extensive, with respect to the longitudinal axis of the bit, at the azimuthal position of the particular blade, either a pass through circle or a drill circle. The drill circle and the longitudinal axis are substantially coaxial. The axis of the pass-through circle is offset from the longitudinal axis and defines an arcuate section wherein the pass-through circle extends laterally from the longitudinal axis past the drill circle. The leading edge cutters on the reaming blades are, as a result of this selected shape of the reaming blades, located radially inward of the trailing edge of the reaming blades with respect to the pass through circle where the reaming blades conform to the drill circle (in the arcuate section). This provides that the drill bit can pass through the casing, which has a diameter of about the pass-through diameter, without damaging the inserts on the reaming blades. When the bit fully penetrates the float equipment and exits the casing, the bit is then rotated about the longitudinal axis and then drills a hole, in the earth formations beyond the casing, which has the drill diameter.